Self-focusing electron beam apparatus



Nov. 16, 1965 H. STAUFFER SELF-FOCUSING ELECTRON BEAM APPARATUS 2Sheets-Sheet 1 Filed Dec. 27, 1962 Beam Cor/en t [)7 vehtor: g/2n HStay/fer; 49 71* His A 6' t orngy.

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HAS A z? tor/7% United States Patent 3 218,431 SELF-FQCUSING ELECTRONBEAM APPARATUS Lynn H. Stauffer, Pattersonville, N.Y., assignor toGeneral Electric Company, a corporation of New York Filed Dec. 27, 1962,Ser. No. 247,730 12 Claims. (Cl. 219-121) My invention relates tocertain improvements in electron beam irradiation apparatus of thegaseous beam type whereby the intensity of the beam may be moreeffectively controlled than heretofore.

One form of gaseous electron beam apparatus, as now known, comprises ahollow cathode structure having perforated sidewalls arranged within ahousing filled with a low pressure ionizable gas and maintained at highnegative potential relative to the housing. In operation, a plasma ofionizable gas forms within the cathode from which, due to said potentialand interaction of the plasma and side walls of the cathode structure, abeam of electrons is emitted from the cathode through an aperture formedin a wall thereof. This beam may be caused to fall upon material to beirradiated which may be at the more positive potential of the housingand exposed to the ionizable gas.

One shortcoming of this apparatus is the inability to preventcontamination of the cathode by gases or vapors which may be generatedby excessive gassing of the material being irradiated, therebydecreasing the life of the cathode.

Another shortcoming in such apparatus is the inability to control theintensity or magnitude of current in the beam without affecting itsfocus and without variation of said high potential or gas pressure.

An object of my invention is to obviate these shortcomings and toprovide means to vary the intensity of the beam over a wide rangewithout affecting its focus and independently of the negative operatingvoltage and gas pressure.

A further object of my invention is to effect such control by varying arelatively small voltage applied between a control electrode structurearranged in accord with my invention and the cathode structure tothereby effect such control largely with facility characteristic ofother types of discharge devices.

In accord with my invention, a control electrode is arranged Within thecathode structure, shaped in general conformity to that structure andspaced from its side walls and insulated therefrom. This controlelectrode may be connected to a suitable low voltage source by which itspotential relative to the cathode structure may be varied in either thepositive or negative direction over a desired range thereby to vary thecurrent in the beam. I believe that the operation of the controlelectrode so arranged is to control the electron density in the plasmawhich is enclosed by the control electrode, thereby controlling thecurrent in the beam issuing from the plasma through aligned apertures inboth the control electrode and cathode structure. The electron beam thusproduced is especially useful in high quality metal working such ascutting, welding, brazing and also in fusing dissimilar materialsincluding refractory substances such as porcelain to tantalum whereinhigh processing temperatures in the order of 3000 C. are required.

The features of my invention which I desire to protect herein arepointed out with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the accompanying drawings, wherein like parts in each of theseveral figures are identified by the same reference character, andwherein:

FIGURE 1 is an elevation view, partly in section, illustrating anelectron beam irradiation apparatus constructed in accordance with myinvention;

FIGURE 2 is an elevation view, partly in section, illustrating a firstembodiment of an electron beam irradiation apparatus employing a controlelectrode and electrostatic beam focusing means;

FIGURE 3 shows a series of current versus control electrode voltagecharacteristic curves illustrating the control obtained in accordancewith my invention;

FIGURE 4 is a detail view of a first embodiment of the cathode andcontrol electrode structure arrangement shown in FIGURE 2;

FIGURE 5 is a detail view of a second embodiment of a cathode andcontrol electrode structure arrangement;

FIGURE 6 is an elevation view, partly in section, illustrating a secondembodiment of an electron beam irradiation apparatus employing a controlelectrode and electromagnetic beam focusing means;

FIGURE 7 is a detail view of a cathode and control electrode structurearrangement whereby an electron beam having a rectangular cross sectionmay be produced; and

FIGURE 8 is a cross sectional view of the cathode and control electrodestructure arrangement taken on the plane of line 8-3 of FIGURE 7.

Referring particularly to the apparatus illustrated in FIGURE 1, thereis shown a housing designated as a whole by numeral 1, preferably ofcylindrical shape although other forms may also be employed. Housing 1is constructed of an electrically conductive material that is nonporousand preferably metallic and as illustrated, comprises a top end plate 2,hollow cylindrical wall 3, and bottom end plate 4. Top end plate 2 isjoined to cylindrical wall 3 by any Well known metal to metal joiningmethod, the particular method not being critical since a high vacuum isnot required therein. Bottom end plate 4 or a lower section ofcylindrical wall 3 is made removable to facilitate the insertion andwithdrawal of material 5 being processed in container 6 which rests onbottom end plate 4. Container 6 may be made of copper or other suitablegood electrically conductive and good heat conductive material and theanode of the apparatus is then considered as comprising housing 1 andcontainer 6.

The electron source consists of a hollow cathode structure 7, preferablyin the form of a cylinder although other shapes may be employed, with anaperture 9 in the center of a bottom end wall thereof wherefrom anelectron beam is emitted by nonthermionic means in a manner to bedescribed in detail hereinafter. This hollow cathode structure 7 isconstructed from an electrically conductive material which must becapable of being formed, have a relatively high melting point to avoidmelting at the red heat temperature to which the cathode may besubjected at high beam intensities even though no heat source as such isutilized, preferably not emit significant amounts of gas at thistemperature, and have relatively high secondary electron emissioncharacteristics. A preferred embodiment of the cathode is constructedfrom a perforated sheet or mesh of molybdenum, although other surfacescharacterized by a number of small opening therethrough are alsoappropriate. The cathode may also be constructed of stainless steel orcopper by way of further example. Although the top and bottom end wallsof the cathode may be constructed of solid electrically conductivematerial, they are preferably made of the same perforated material asthe cylindrical wall thereof.

A power supply line connected to terminals 10 supplies an adjustablevoltage of relatively high negative direct current potential to cathode7 relative to housing 1 by means of conductor 11 and cathode stem 12connected thereto. The positive or ground side of the power supply lineis connected to the anode or housing 1. Cathode stem 12 is insulatedfrom top end plate 2 by means of insulating bushing 13 and comprises ahollow tubular electrically conductive member, preferably made ofstainless steel, that supports cathode 7 and positions it within housing1, and also provides a passage means for an electrical conductor to bedescribed hereinafter in relation to FIGURE 2. A hollow tubularelectrically conductive shield 14, concentric with cathode stem 12 andspaced therefrom is positioned along the upper portion of stem 12contained within housing 1 and in good electrical contact therewith toprevent long-path discharge between the cathode stem and top end plate2. The high voltage supplied at terminals is adjustable up to kilovoltsor more and may be provided by a power supply consisting of adjustablealternating current voltage source 15, whose voltage is increased bymeans of step-up transformer 16 and then converted to a filtered directcurrent voltage by means of rectifiers 17 and a conventional filternetwork illustrated as a whole by numeral 18 and which may includeresistors, inductors, and capacitors.

A suitable ionizable gas, such as argon or helium, is introduced intothe interior of the enclosure within housing 1 that surrounds cathode 7through passage means 19 which may pass through top end plate 2 or theupper portion of side wall 3. Passage means 19 is connected to a gassupply 20 through valve 21 which regulates the rate of gas flow intohousing 1. A partitioning member 22, nonporous to the gaseous medium andwhich may be made of the same material as housing 1, separates thehousing into two enclosures, the first or upper enclosure enclosing thecathode and the second or lower enclosure containing the material beingirradiated and processed by an electron beam emitted by the cathode. Anaperture 23, within partitioning member 22 and aligned with cathodeaperture 9, is of size sufiicient merely to permit passage of theelectron beam therethrough and insufficient in size to permitobjectionable passage of any gases or vapors which may be generated byexcessive gassing of the irradiated material 5 in the lower orprocessing enclosure. A second passage means 24 located in the lowerenclosure provides, by virtue of its large size, a low impedance exitfor this generated gas and thus aids in maintaining a desired gaspressure within the upper or cathode enclosure, and is connected to asuitable exhaust pumping device 25 through throttle valve 26. Thus,possible contamination of the cathode by undesired gases generated bythe irradiated material is largely prevented by employing the particularpartitioning member described. Further, the partitioning member aids inmain taining a desired low gas pressure in the cathode enclosure andthereby maintain the electron beam in a collimated mode.

One theory for explaining the principle of electron beam formation andejection from a hollow perforated cathode is as follows: The interior ofthe cathode cavity comprises a glowing body of plasma or ionized gas,separated from the cathode walls by a less luminous sheath which isbounded by said walls. Positive ions and free electrons comprise mainconstituents of the plasma. The potential distribution inside thecathode comprises equipotential surfaces that extend through cathodeaperture 9, thereby crowding together at the aperture and effecting ahigh voltage gradient which extracts electrons from the internal plasmaand initiates an electron beam. An external glow discharge or plasmasurrounding the cathode determines an ionized region of low voltagedrop, separated from the cathode by a sheath or dark space in which alarge voltage drop occurs. Since the cathode potential may be 5 to 20kv. negative with respect to the anode, positive ions are drawn from theexternal plasma and accelerate across the dark space to impinge on theouter surface of the cathode or p in through i116 interstices of theperforated cathode. These ions possess several thousand electron voltsof kinetic energy and they may release large numbers of electrons bysecondary electron emission due to impact with the cathode surface, byionizing collisions with the gas, and by excitation of atoms by indirectprocesses which emit photons which, in turn, give rise to photoelectronsat the cathode.

The positive ions drawn from the external plasma and impinging on theouter surface of the cathode release secondary electrons which arerepelled from the cathode and ionize the gas in their path by collision.This ionization maintains the external plasma which is the source ofpositive ions.

Many of the positive ions drawn from the external plasma and passingthrough the interstices of the perforated cathode strike the innercathode surface and thereat generate secondary electrons. Each positiveion may generate several secondary electrons which are drawn into theinternal plasma and thence from cathode aperture 9 by the strongpositive potential gradient existing thereat. If the external fieldstrength is greater than that inside the cathode, the aperture has aconverging action on the emerging electron stream, thus explaining whybeam collimation is voltage dependent. Due to the continuous extractionof electrons at the cathode aperture, the internal plasma body assumes apositive potential which expels positive ions to the cathode walls.These expelled positive ions, together with incoming secondaryelectrons, attempt to maintain equilibrium of electrical charge of theinternal plasma against the outward drain due to the electron beam. Theelectron beam is collimated by what may be described as a gas-focusingprocess when both the gas pressure and cathode potential relative to theanode are maintained Within a particular critical range dependent on thegaseous medium utilized. Any inert gas or metallic vapor, as well ashydrogen, may be employed as the gaseous medium contained within theupper enclosure and for best performance from the standpoint of smallelectron beam cross section and large beam current, a cathode diameterto cathode apcr ture ratio of approximately 4 to 1 is employed.

Control of the beam intensity, that is, the total current within theelectron beam, over a substantial range of beam current may be obtainedby simultaneously adjusting the gas pressure and cathode to anodepotential. However, the beam is not self-focusing, that is, the beamintensity is not controllable independently of the focus, thus, asignificant change in beam intensity produces a poorly focused beam andin the extreme case may cause the cathode discharge to pass out of thebeam mode and become a diffuse glow discharge. It can be appreciatedthat for applications such as welding, or cutting, an electron beamhaving a high power concentration, that is, a finely focused orcollimated beam is generally desired over a wide range of beam intensitycontrol.

The intensity of the beam may be controlled without affecting its focusand without adjustment of the gas pressure and high cathode-to-anodepotential, that is, it may be made to maintain its self-focusedcondition by positioning a control electrode structure shaped in generalconformity to the cathode, within said cathode and applying anadjustable potential between said control electrode and cathode. Controlelectrode 27 is positioned substantially centrally of the cathode andspaced therefrom whereby it is electrically insulated from the cathode.The spacing between cathode and control electrode is preferablyapproximately lO percent of the cathode diameter, although thisdimension may be varied over a considerable range and is not recited asa limitation. Control electrode 27 is provided with an aperture 28 inthe lower end thereof that is substantially concentric with cathodeaperture 9. The control electrode structure or grid is preferablyconstructed of very fine wire and preferably has a sur facecharacterized by a greater open area per unit area of surface than thecathode. However, the control electrode structure is not limited bythese characteristics, and a smaller open surface and thick wireconstruction will also control beam intensity, although lesseffectively. The control electrode may be fabricated from similarmaterial comprising the cathode. The top and bottom end walls of thecontrol electrode structure are preferably constructed of the samerelatively open mesh surface as the cylindrical wall thereof to minimizepositive ion interception which may occur from all directions. In thealternative, the top and bottom end walls of the control electrode maybe completely open. Tubular cathode stem 12 provides a passage means foran electrical conductor 29 that supplies control voltage to controlelectrode 27. Conductor 29 passes through cathode stem 12 and iselectrically insulated therefrom by insulation material which isappropriate to the potential applied between the cathode and anode andalso forms a gas-tight seal. Conductor 29 is preferably made of heavywire to support the control electrode and maintain its positionconcentrically within the cathode. One end of conductor 29 iselectrically connected to a movable arm on potentiometer 30 and arelatively low direct current voltage of approximately 100 voltssupplied to terminals 31 is applied across potentiometer 39 to obtain avariable positive or negative potential on the control electrode withrespect to the cathode.

The effect of control electrode structure 27 is to control the electrondensity in the plasma, the plasma being enclosed by the electrodestructure and thereby controlling the current in the beam issuing fromthe plasma through the aligned apertures in both the control electrodeand cathode. iiaintaining the control electrode potential egative withrespect to the cathode repels secondary electrons emerging from theinner cathode surface back toward the surface, thus reducing the supplyof electrons available to the beam. Maintaining the control electrode atthe same potential as the cathode produces little effect since therelatively open control electrode structure renders it highlytransparent to electrons. However, maintaining the control electrodepotential positive with respect to the cathode assists the transfer ofsecondary electrons into the internal plasma which maintains the beamand thereby increases the beam intensity.

Referring particularly to FIGURE 3, it is observed that as the controlelectrode potential (grid volts) is made increasingly positive withrespect to the cathode, the beam current or intensity increases to amaximum and then decreases. This latter effect is believed to be relatedto the influence of the electric field between the control electrode andcathode on the distribution of positive ions within the cathode. It maybe seen from FIGURE 3 that substantially the full range of beam currentmay be controlled by varying the potential between control electrode andcathode approximately plus or minus volts. This means of controlling thebeam current is very efiicient since the ratio of watts beam powercontrolled to watts grid or control electrode power required for thiscontrol may be approximately 2000 to l or greater. The particular curvesillustrated in FIGURE 3 were obtained for a cathode-control electrodearrangement contained in a gaseous medium of argon at 7 microns pressureand a athode-to-anode potential of 11.0 kv.

The specific cathode and control electrode structure arrangement fromwhich the curves of FIGURE 3 were obtained is shown in the detail viewof FIGURE 4. Cathode 7 is shown in the broken section as comprising ahollow perforated cylindrical chamber having a diameter of 1% inches, alength of 1 /2 inches, and constructed of perforated stainless steelhaving 0.2 mm. holes with holes per centimeter. Control electrode 27comprises a hollow cylindrical body having a diameter of Va inch, alength of IVs inches, and constructed of a mesh structure having 32 meshper inch and made of 0.005 molybdenum wire. It should be understood thatthe control electrode may also comprise a perforated structure, or bothcathode and control electrode may comprise a mesh structure or otherrelatively open grid-like structures, and these configurations wouldproduce characteristic curves similar to those of FIGURE 3 if theabove-recited dimensions were maintained. Electrical insulation 32,appropriate to the cathode-to-anode potential, insulates the controlelectrode from the cathode and acts as a gas seal and a further supportfor the control electrode structure.

Referring back to FIGURE 2, a second stage of exhaust pumping isemployed as distinguished from the single stage in FIGURE 1. Formaterials 5 giving off little or no gas during irradiation, very littlepumping is necessary and the pumping arrangement illustrated in FIGURE 1is satisfactory. However, for liquids in film or spray form which areencountered in the sterilization of drug products, chemical synthesis ofcompounds, and polymerization, or for solids having high vapor pressure,a second stage of exhaust pumping as illustrated in FIG- URE 2 isneeded. Thus, partitioning member 33 with aperture 34 therein, alignedwith the cathode and control electrode apertures and aperture 23,defines an intermediate enclosure contained between partitioning members22 and 33 and is provided with an exhaust pumping means (not shown)through passage means 35. In this particular application, the exhaustpumping through passage 35 primarily determines the pressure of the gasintroduced through passage means 19 within the upper or cathodeenclosure, although it also acts as a further impediment, alon with thesmall dimensions of apertures 23 and 34, to the flow of contaminatinggas or vapor from material 5 into the upper enclosure.

An additional feature of the apparatus illustrated in FIGURE 2 is theintroduction of a second gaseous medium into the bottom or processingenclosure through passage means 36. In applications requiring theirradiation of material 5 ina gaseous atmosphere different from the gascontained in the upper enclosure, as in the case of nitriding steel, thedesired gas is introduced into the bottom enclosure and exhaustedthrough passage means 24, the gas supply, pumping and valve devices notbeing shown. Since apertures 23 and 34 present a high impedance for anygas passage, the second gaseous medium is primarily contained within thebottom enclosure and any slight amount which passes through aperture 23is exhausted through passage means 35.

An independent focus adjustment of the electron beam may be obtained byelectrically insulating partitioning member 33 from the wall of housing1 by means of insulator 37 and applying an adjustable potential as shownto member 33 by means of conductor 38. Conductor 33 is connected topartitioning member 33 and passes through the housing wall by means ofinsulating bushing 39 due to the relatively high negative potentialimpressed on number 33 relative to housing 1. Conductor 38 is connectedto the movable arm of potentiometer 40 which in turn is connected acrossthe cathode high voltage supply terminals 10. An electrically conductivecylinder 41 having open ends may be connected to member 33 at aperture34 to provide a more efiicient electrostatic focusing or defocusing ofthe electron beam. The electrostatic focusing can be adjusted wherebythe beam possesses its smallest cross section in passing throughaperture 23, thereby permitting a smaller aperture to be used therentand also, to control the focus of the beam as it comes in contact withthe irradiated material 5. Thus, for welding operations, theelectrostatic focusing is adjusted to define a very finely focused orhigh power density beam on the work being welded whereas for a chemicalprocess requiring large area irradiation, the beam is substantiallydefocused. Ballast resistor 42 is connected in the cathode power supplyline for cathode voltage stabilization, it being understood that asimilar resistor would likely be employed in FIGURES l and 6, althoughnot illustrated therein.

A second embodiment of a cathode control electrode structure arrangementconstructed in accordance with my invention is illustrated in FIGURE 5.In this particular arrangement, cathode 7 is formed of a mesh structureand the control electrode 27 comprises a helically wound coil of wire.The materials from which these structures are constructed may be thesame as recited for the arrangement in FIGURE 4.

FIGURE 6 illustrates a second embodiment of an electron beam apparatuscontaining a cathode and control electrode structure arrangement. Inthis particular configuration, the control electrode voltage source isconnected to terminals 31 and therefrom to potentiometer 30 by means ofreversing switch 43. Reversing switch 43 in a first position as shown inFIGURE 6 renders the control electrode potential negative with respectto the cathode. In a second position, illustrated as the extreme lowerone, the control electrode potential is rendered positive with respectto the cathode. Switch 43 may also be provided with a neutral positionas shown whereby potentiometer 30 is disconnected from terminals 31 andpotentiometer 30 now acts as a rheostat, and the control electrodepotential may be controlled by a self-biasing arrangement that makes thecontrol electrode positive with respect to the cathode by increasing theresistance in series therewith. In any of the switch positions, thecontrol electrode may be adjusted to the same potential as the cathodeby setting the movable arm of potentiometer 30 to the extreme lowerposition.

Another feature of the apparatus illustrated in FIGURE 6 is thearrangement of the inlet and exhaust passage means passing through thewalls of housing I. The entrance passage means 44 for the gaseous mediumto be contained by the upper enclosure is located in the housing wall ofthe intermediate enclosure. With this arrangement, aperture 34 is madeof size sufiicient for passage of both the electron beam and the gaseousmedium. A passage means 45 provides a gas exhaust from the upperenclosure therein. It is to be understood that entrance passage 44, andexhaust passages 45 and 24 are connected to suitable valves and pumpingdevices to maintain the desired low gas pressure within the upperenclosure. An advantage of this gas distribution arrangement is that thehigher pressure of the gaseous medium within the intermediate enclosurefurther impedes any undesired gas which may be generated in the lowerenclosure from passing into the upper enclosure.

An added feature of the apparatus disclosed in FIG- URE 6 is the use ofmagnetic focusing coils to control the cross section of the electronbeam both in passing through aperture 23 in the lower partitioningmember 22 and also at the material being irradiated by said beam. Inthis case, the partitioning members must be constructed of nonmagneticmaterial. A first electromagnetic coil 46, which may simply be wound ona nonmagnetic spool in concentric relationship to the electron beam andspaced therefrom, is adapted to focus the beam to its smallest crosssection as it passes through aperture 23. A second electromagneticfocusing coil 47, similar in construction to coil 46 is positioned inthe lower enclosure to control the now diverging electron beam into afinally focused spot on the material 5 being irradiated, or in thealternative, further defocus the beam to provide Wide area irradiation.Although coils 46 and 47 may each be connected across a power supplyhaving a high voltage and low current output, I prefer to employ a powersupply having a low voltage and high current output whereby theconductors 48 and 49 joining the ends of each coil, respectively, may bebrought out through the side walls of housing 1 by merely employing lowvoltage insulation surrounding said conductors. Electron beam powerdensities of 10,000 kw./in. or greater may be obtained by the apparatushereinabove disclosed.

The cross-sectional shape of the electron beam generated by the cathodeand the control electrode structures of my invention is determinedprimarily by the geometry of the apertures 9 and 23 in the respectivebottom walls thereof. For many applications, these apertures arecircular in shape and thereby cause the generation of an electron beamhaving a circular cross section. However, in applications such as heattreating a moving sheet of metal, it is desirable to provide a longrectangular beam of concentrated electrons whereby the long dimen sionthereof may irradiate the full width of the sheet as it passes thereby.To produce the desired electron beam having a long rectangular crosssection, apertures 9 and 28 are located in alignment on a cylindricalside of the cathode and control electrode structures, respectively, asillustrated in FIGURE 7. The rectangular shaped aperture is formedtherethrough with the long dimension of the rectangle being in the axialdirection of the cylindrical structures. The electron beam emittedthrough apertures 9 and 28 thus has a desired long rectangular crosssection.

FIGURE 8 is a plan view on the plane of line 8-8 of FIGURE 7 andillustrates the relative width or short dimension of rectangularapertures 9 and 28, Whereas FIGURE 7 indicates their long dimension.

From the foregoing description, it can be appreciated that my inventionmakes available a new apparatus for irradiating materials by means of anelectron beam wherein gases or vapors produced by the material beingirradiated are prevented from contaminating or disturbing the focusedcondition of the beam emitting cathode which is contained in anenclosure separated from the enclosure containing the material beingirradiated. It is to be understood that the number of enclosures is notlimited and is determined primarily by the controlled atmospheres inwhich the cathode and irradiated material are to opcrate. The electronbeam is nonthermionically emitted through an aperture in a hollowperforated cathode by interaction of an ionizable gaseous mediummaintained at low pressure and a high negative potential maintained atthe cathode relative to the anode. This nonthermionic emission permitsthe cathode to function effectively at low temperatures. The electronbeam may maintain selffocusing by providing a control electrode withinthe cathode. This self-focusing feature permits control of the beam overa Wide range of beam power in a simple manner by adjusting a relativelylow potential applied between the cathode and the control electrode.Since this control is obtained independently of the beam focus whichremains fixed, no adjustment in the pressure of the gaseous medium or inthe relatively high potential applied to the cathode relative to theanode, to restore focus, is required. The self-focusing feature, underappropriate conditions of cathode voltage and gas pressure, also permitselimination of the conventional beam focusing techniques normallyemployed in high vacuum, thermionically emitting electrode electron beamgenerators, although they may be employed to provide an independentfocus adjustment of the beam as dictated by the particular applicationemploying the beam irradiation. Finally, the apparatus may be used forirradiating materials in controlled environments including gaseousmediums different from the gas contained in the cathode enclosure, andmay even be used for irradiation in ambient atmospheric condltions, inwhich case, the processing enclosure would be left open to the ambientair. The irradiated material may be positioned in the last enclosure oran intermediate one.

Having described a new and improved apparatus for generating an electronbeam in a low pressure gaseous medium wherein the beam may be controlledover a wide range of beam intensity independently of the beam focus, itis believed obvious that modifications and variations of my inventionare possible in the light of the above teachings. Thus, alternatingcurrent power or any combination of direct and alternating current powermay be applied to the cathode and control electrode structures to obtaina controlled pulsating electron beam. It is,

therefore, to be understood that changes may be made in the particularembodiments as described which are within the full intended scope of theinvention as defined by the following claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electron beam generating apparatus comprising:

a housing,

means for defining a plurality of enclosures within said housing, and

a hollow cathode structure having a surface characterized by a number ofsmall openings therethrough, said cathode disposed within a first ofsaid enclosures, means for introducing a low pressure ionizable gaseousmedium within said first enclosure, means for operating said cathode ata high negative potential relative to the housing sufiicient to producea plasma within said cathode, said cathode and said enclosure definingmeans each having an aperture, the apertures aligned with respect toeach other whereby an electron beam issuing from the plasma passesthrough said apertures into another of said enclosures, said otherenclosure adapted to utilize said beam which utilization may generate anundesired gaseous medium, the aperture in said enclosure defining meansbeing of size sufficient for passage of the electron beam andinsuflicient for passage of objectionable amounts of the undesiredgaseous medium into said first enclosure.

2. An electron beam generating apparatus comprising:

a housing,

means for partitioning said housing into a plurality of enclosures,

a hollow cathode structure having perforated sides, said cathodepositioned in a first of said enclosures and electrically insulatedtherefrom, means for introducing a low pressure ionizable gaseous mediumwithin said first enclosure, means for providing a high negativepotential on said cathode relative to said housing whereby a collimatedelectron beam issues from a plasma generated within and enclosed by thecathode, said cathode having an aperture through which said beam passesinto said first enclosure,

said housing partitioning means have an aperture aligned with saidcathode aperture whereby said beam passes into another of saidenclosures, said other enclosure adapted to utilize said beam whichutilization may produce a second gaseous medium, the aperture in saidhousing partitioning,means being of size sufiicient for passage of saidbeam therethrough and insufiicient for passage of objectionable flow ofsecond gaseous medium into said first enclosure,

means for providing entrance and exit passages for the first and secondgaseous mediums, and

means for controlling the intensity of the electron beam independentlyof the focus thereof.

3. The apparatus set forth in claim 2 including:

a passage means disposed in a wall of said other enclosure forintroducing a controlled gaseous atmosphere within said other enclosurewhereby material to be irradiated by said electron beam is subjected tosaid controlled gaseous atmosphere.

4. The combination of a hollow cathode structure having a perforatedsurface, means for providing a low pressure ionizable gaseous medium, tothe cathode, means for operating the cathode at a high potentialrelative to the enclosure of said medium sufficient to produce a plasmawithin the cathode, said cathode having an aperture through which anelectron beam issues from the plasma, and

means positioned within said cathode and electrically insulatedtherefrom for controlling the intensity of the electron beamindependently of the focus thereof.

5. The combination of a hollow cathode structure having perforatedsides, means for providing a low pressure ionizable gaseous medium tothe cathode, means for operating the cathode at a high negativepotential relative to the enclosure of said medium sufiicient to producea plasma within the cathode, said cathode having an aperture thorughwhich an electron beam issues from the plasma, and

a control electrode structure positioned within said cathode structureand arranged to enclose said plasma, said control electrode having anaperture aligned with said cathode aperture through which said beamissues whereby the intensity of current in said beam may be varied byvarying the potential between said control electrode and cathodestructures.

6. The combination of a hollow cathode structure having a surfacecharacterized by a number of small openings, said cathode positionedwithin an enclosure adapted to contain a low pressure ionizable gaseousmedium, means for providing a low pressure ionizable gaseous mediumwithin the enclosure, means for operating said cathode at a highnegative potential relative to the enclosure sufficient to generate aplasma within the cathode, said cathode having an aperture through whichan electron beam issues from the plasma to the exterior of said cathode,and

a hollow control electrode shaped in general conformity to the cathodestructure and having a surface characterized by a grid-like structure,said control electrode positioned within and substantially centrally ofsaid cathode and electrically insulated therefrom, said controlelectrode enclosing said plasma and having an aperture aligned with saidcathode aperture through which said beam passes and whereby themagnitude of current in said beam may be varied independently of thebeam focus by varying a low potential between said control electrode andcathode.

7. The combination set forth in claim 6 wherein said hollow controlelectrode comprises a mesh structure.

8. The combination set forth in claim 6 wherein said hollow controlelectrode comprises a helically wound coil of wire.

9. The combination set forth in claim 6 wherein said hollow controlelectrode comprises a perforated structure.

10. In an apparatus for irradiating a material with an electron beamhaving a generally rectangular cross section the combination of,

a hollow cylindrical cathode structure having a surface characterized bya large number of small openings, said cathode electrically insulatedfrom and positioned within an enclosure, means for supplying a lowpressure ionizable gaseous medium within the enclosure,

an electrical conductor connected from said cathode through saidenclosure and insulated therefrom, means for supplying a high negativepotential to said cathode relative to said enclosure by means of saidelectrical conductor whereby an interaction of the gaseous medium andpotential produces a plasma contained within said cathode, said cathodehaving a long rectangular shaped aperture in the cylindrical sidethereof, the long dimension of said aperture being in the axialdirection of the cylindrical structure whereby an electron beam having arectangular cross section may issue from the plasma and pass throughsaid aperture,

a hollow control grid structure having a mesh surface, said control gridpositioned within and substantially centrally of said cathode andenclosing said plasma, said control grid electrically insulated fromsaid cathode and having a rectangular shaped aperture aligned with saidcathode aperture wherethrough said rectangular shaped beam passes, and

a second electrical conductor connected from said control grid throughsaid enclosure and insulated from said cathode and enclosure, means forsupplying a variable low potential to said control grid relative to saidcathode by means of said second electrical conductor and thereby controlthe intensity of the rectangular cross section beam currentindependently of the beam focus.

It. An electron beam welding apparatus comprising:

a housing,

a pair of nonporous members for partitioning said hous- ,ing into threeenclosures,

a hollow cathode structure having perforated sides, said cathodepositioned within a first of said enclosures and electrically insulatedtherefrom, said cathode having an aperture through which an electronbeam may pass,

means for supplying a controllable high negative potential on saidcathode relative to said housing,

a hollow control electrode structure shaped in general conformity to thecathode structure and having a surface characterized by a greater openarea per unit of surface than said cathode, said control electrodepositioned within and substantially centrally of said cathode andelectrically insulated therefrom and having an aperture aligned withcathode aperture,

a first passage means connected to a source of low pressure ionizablegaseous medium, said first passage means disposed in a wall of saidfirst enclosure whereby a low pressure ionizable gaseous medium may beintroduced therein and an interaction of the gaseous medium and negativepotential between cathode and housing may produce a plasma enclosed bysaid control electrode and emission of a collimated electron beam fromthe plasma through the grid and cathode apertures,

means for supplying an adjustable low potential on said controlelectrode relative to said cathode and thereby provide control of theintensity of the electron beam independently of the beam focus,

said partitioning members each having an aperture aligned with saidcathode and grid apertures whereby said electron beam passes through asecond of said enclosures and into a third enclosure adapted to utilizethe power within said electron beam in a welding operation, theapertures in said members being of size suflicient for passage of saidbeam therethrough and insufficient for appreciable passage of a. secondgaseous medium which may be generated in said third enclosure,

a second passage means disposed in a wall of said second enclosure forexhausting gas within said second enclosure and thereby maintaining theionizable gas in said first enclosure within a relatively narrow lowpressure range and further impeding passage of the second gaseous mediuminto said first enclosure,

a third passage means disposed in a wall of said third enclosure forexhausting said second gaseous medium which may be generated by materialbeing welded by said electron beam in said third enclosure,

a first electromagnetic coil positioned within said sec ond enclosuresubstantially concentric to said electron beam and spaced therefromwhereby the electron beam may be controllably focussed to a very smallcross section as it passes through the aperture in the partioningmembers separating the second and third enclosures, and

a second electromagnetic coil positioned in said third enclosuresubstantially concentric to said electron beam and spaced therefrom forcontrolling the focus of the electron beam on material being welded insaid third enclosure, said first and second electromagnetic coils havingtheir ends passing through the sides of said housing whereby anadjustable voltage may be impressed across each coil.

12. An electron beam irradiating apparatus comprising:

a housing,

a pair of nonporous partitioning members disposed within said housingwhereby three enclosures are defined therein,

a hollow cathode structure having a surface characterized by a number ofsmall openings, said cathode positioned within a first of saidenclosures and electrically insulated therefrom, said cathode having anaperture through which an electron beam may pass,

a hollow control grid structure positioned within and substantiallycentrally of said cathode and electrically insulated therefrom, saidgrid having an aperture aligned with said cathode aperture,

means for supplying a high negative potential to said cathode relativetosaid housing,

means disposed in a wall of a second of said enclosures for introducinga low pressure gaseous medium into said second enclosure,

passage means disposed in a wall of said first enclosure for exhaustinga portion of said gaseous medium within said first enclosure and therebymaintaining said gaseous medium within a predetermined low pressurerange,

the first partitioning member separating said first and secondenclosures having an aperture aligned with said cathode and gridapertures and suflicient in size for passage of an electron beam emittedby a plasma generated interior of said grid by interaction of saidgaseous medium and high negative cathode potential and sufiicient insize for passage of said gaseous medium from said second enclosure intosaid first enclosure,

means for operating said grid at an adjustable low potential relative tosaid cathode and thereby controlling the magnitude of current in saidbeam independently of the beam focus,

the second partitioning member separating said second and thirdenclosures having an aperture aligned with said cathode, grid and firstpartitioning member apertures and sufficient in size for passage of saidelectron beam and insufficient in size for appreciable passage of anundesired gas which may be generated in said third enclosure,

passage means disposed in a wall of said third enclosure for exhaustingundesired gas generated by material irradiated by said electron beamwithin said third enclosure, and

said first partitioning member electrically insulated from the walls ofsaid housing and provided with connections whereby an adjustable highnegative potential may be impressed on said first partitioning memberrelative to said housing to provide electrostatic control of the beamfocus on the material being irradiated in said third enclosure.

References Cited by the Examiner UNITED STATES PATENTS 2,899,556 8/1959Schopper et al. 3,009,050 11/1961 Steigerwald.

RICHARD M. WOOD, Primary Examiner.

JOSEPH V. TRUHE, Examiner.

1. AN ELECTRON BEAM GENERATING APPARATUS COMPRISING: A HOUSING, MEANS FOR DEFINING A PLURALITY OF ELECTRODES WITHIN SAID HOUSING, AND A HOLLOW CATHODE STRUCTURE HAVING A SURFACE CHARACTERIZED BY A NUMBER OF SMALL OPENINGS THERETHROUGH, SAID CATHODE DISPOSED WITHIN A FIRST OF SAID ENCLOSURES, MEANS FOR INTRODUCING A LOW PRESSURE IONIZABLE GASEOUS MEDIUM WITHIN SAID FIRST ENCLOSURE, MEASNS FOR OPERATING SAID CATHODE AT A HIGH NEGATIVE POTENTIAL RELATIVE TO THE HOUSING SUFFICIENT TO PRODUCE A PLASMA WITHIN SAID CATHODE, SAID CATHODE AND SAID ENCLOSURE DEFINING MEANS EACH HAVING AN APERTURE, THE APERTURES ALIGNED WITH RESPECT TO EACH OTHER WHEREBY AN ELECTRON BEAM ISSUING FROM THE PLASMA PASSES THROUGH SAID APERTURES INTO ANOTHER OF SAID ENCLOSURES, SAID OTHER ENCLOSURE ADAPTED TO UTILIZE SAID BEAM WHICH UTILIZATION MAY GENERATE AN UNDESIRED GASEOUS MEDIUM, THE APERTURE IN SAID ENCLOSURE DEFINING MEANS BEING OF SIZE SUFFICIENT FOR PASSAGE OF THE ELECTRON BEAM AND INSUFFICIENT FOR PASSAGE OF OBJECTIONABLE AMOUNTS OF THE UNDESIRED GASEOUS MEDIUM INTO SAID FIRST ENCLOSURE. 